Piezoceramic multilayer actuator (see FIG. 1) in the state of the art consist of stacked thin layers of piezoelectrically active material 2, e.g., lead zirconate titanate (PZT) with conductive internal electrodes 7 arranged between them, which run alternately to the surface of the actuator. External electrodes 3 and 4 connect these internal electrodes together. The internal electrodes are thereby connected electrically parallel and in a combination of two sets constituting the two terminal poles of the actuator. If an electrical voltage is applied to the terminal poles it is transferred parallel to all of the internal electrodes and cause an electrical field in all layers of the active material which is thereby deformed mechanically. The sum of all these mechanical deformations is available at the end surfaces of the actuator as useful expansion 6 and/or force.
Piezoceramic multilayer actuators are, in the state of the art, made in the form of monoliths, that is, the active material in the form of so-called green films are provided by screen printing methods with internal electrodes prior to sintering, pressed into actuator stacks, pyrolyzed and then sintered, resulting in the monolithic actuator.
A metal ground coating 3 is applied to the actuator stack 1 in the area of the brought-out internal electrodes 7, e.g., by screen printing with metal paste. This metal ground coating is strengthened by applying a metal material 4, e.g, by soldering on an embossed plate or a wire mesh. The electrical connecting wire 5 is soldered onto this strengthened coating.
The structure and the manufacture of such actuators and external electrodes is thoroughly described, e.g., in DE 33 30 538 A1, DE 40 36 287 C2, U.S. Pat. No. 5,281,885, U.S. Pat. No. 4,845,399, U.S. Pat. No. 5,406, 164 and JP 07-226541 A.
The spacing between the internal electrodes 2a of different polarity leading out interdigitally is so small that a flashover would take place at the surface at the working voltage. Therefore the surface is usually covered with an insulating varnish. The fact that thin varnishes offer only an insufficient protection against contact and are mechanically delicate is a disadvantage. Thick varnishes, on the other hand, tend to come loose in operation or when exposed to temperature change. The expression, interdigital areas, is to be understood to refer to the outwardly leading internal electrodes 2a in FIG. 1 between the contacted sides 3 and 4.
Another possibility is to keep the electrodes from reaching all the way to the surface (“buried electrodes”), or to have only the electrodes of one polarity emerge at the surface (“half-buried electrodes”). The disadvantage is that the active surface is diminished and thus the development of the actuator's power is reduced. At the same time these inactive zones result in a cramping of the actuator and are liable to crazing.
Another possibility for protecting the surface is to apply a ceramic film to the green surface of the actuator, sintered them with the actuator in order thus to achieve insulation (DE 10 021 919 A1). A disadvantage in this method is that the outside dimensions of the actuator cannot be reproduced very accurately due to shrinkage during sintering. Moreover, this insulating layer is made relatively thick by the process (typically >50 μm) and thus has disadvantages similar to the two methods described above.
The invention is addressed to the problem of achieving insulation with a ceramic which can be applied after sintering and shaping by grinding and thus can satisfy even severe geometric requirements.
This problem is solved according to the present invention, which relates to a method for the manufacture of piezoelectrical multilayer actuator, wherein thin coats of a piezoceramic material, called green leaves, are applied to the at least one internal electrode, are thus stacked one on the other in a block, that the internal electrodes are brought alternately to opposite faces of the actuator where they are connected together by an external electrode, the actuator green body being sintered and subject to an abrasive shaping and then the ground metallization for the external electrode is applied, characterized in that the areas to be insulated are coated by thick-layer methods with a paste consisting of an inorganic, low-sintering material or material mixture and an organic binder system, and then are subjected to a firing process wherein the layer thickness after sintering is between 1 and 40 μm.